The background art of the invention is described by an example of a hybrid shovel provided with a power unit which is configured by connecting an engine, a generator motor, and a hydraulic pump in series.
In a hybrid shovel, a generator motor and a hydraulic pump are connected to an engine. In the thus-configured hybrid shovel, a hydraulic actuator is driven by hydraulic oil to be ejected from the hydraulic pump. On the other hand, an electric storage device is charged by actuating the generator motor as a power generator, and actuating the generator motor as an electric motor by the electric power from the electric storage device, as necessary, assists the engine.
In a hybrid shovel, there is known a pump device configured such that a coolant passage is formed in a motor housing (see patent literature 1). In the pump device disclosed in patent literature 1, a stator, in other words, the entirety of a generator motor is cooled by heat exchange between a coolant such as water and the stator.
It is desirable to dispose the coolant passage on the outer periphery of the stator in terms of cooling efficiency.
However, in the case where an engine housing and a motor housing are connected to each other on the outer periphery of a stator, it is impossible to provide a coolant passage on the outer periphery of the stator. Accordingly, it is necessary to dispose a coolant passage at a position axially displaced from the outer periphery of the stator (i.e. a connection surface for connecting the engine housing and the motor housing).
In the above configuration, however, the coolant passage may be disposed far away from the stator. As a result, the cooling efficiency may be lowered by an increase of heat transfer loss.
There is proposed a configuration, as disclosed in patent literature 1, in which a stator is disposed at a position axially displaced from a connection surface for connecting an engine housing and a motor housing, and a coolant passage is disposed on the outer periphery of the stator (in other words, both of the stator and the coolant passage are disposed at a position displaced from the connection surface for connecting the engine housing and the motor housing).
The above configuration, however, may increase the size (axial length) of the motor housing in the axis direction thereof. This does not satisfy the demand for shortening the entire length of a power unit as much as possible in order to install the power unit in a limited space of an engine room.